It is well known to package integrated circuits (dies) by placing them on a die-pad area of a lead frame, forming wire connections between electrical contacts of the die and lead fingers of the frame, moulding a resin body around the integrated circuit and wires, and then cutting the lead-fingers to separate them from the lead frame.
There is continuing pressure to increase the number of inputs and outputs of the integrated circuit without increasing its size. One known lead frame 1 is shown in top view in FIG. 1, and a corner portion Z is shown in an expanded view in FIG. 2. The die pad 3 is connected to a support area 5 by elements 7. The lead frame 1 has lead fingers 9 which extend from the support area 5.
During the packaging process, the tip portions 11 of each of these fingers 9 are connected to one or more of the electrical contacts of the integrated circuit (not shown), the integrated circuit is encased in the resin body, and then the lead fingers 9 are cut at the dashed line 13. The tip portions 11 are spaced apart by a distance of only 0.15 mm in the plane of the lead frame.
The packaging process is now explained in more detail with reference to FIG. 3 which is a cross sectional schematic view (not to scale) of a die 15 during the wire bonding process. The die is adhered to the die pad 3. The lead frame 1 also rests on heat blocks 21, and is clamped there by a clamp member 23. The heat blocks 21 form the “paddle” of this clamp-and-paddle arrangement.
Note that the lead fingers 9 as formed of a layer 31, and the tip portion 11 is formed as an upper portion 33 of the layer 31 which projects beyond a lower portion of the layer 31. The manufacturing process of the lead frame includes a chemical etching process applied to the underside of the lead fingers 9 (as viewed in the orientation of FIG. 3) to reduce the thickness of the layer 31 below the portion 33.
A wire 19 located within a capillary tube 17 is ejected as required and bonded to electrical pads on the die 15 and to the tip portions 11 of respective lead fingers 9 by well known techniques, typically involving application of energy in the form of ultrasonic vibrations, to provide wires 25. The tip portions 11 may have a silver coating on at least their upper surface to improve the quality of the bonds.
Once the wire bonding is completed, the heat blocks 21 are removed, and the lead frame 1 is placed onto a horizontal surface in a mould (normally with a foil layer between the lead frame and the horizontal surface), where a resin body is moulded around the die 15 and the tips 11 of the lead fingers 9. After the resin body is formed, the fingers 9 are cut at the line 13 to singulate the integrated circuits and complete the packaging. The packaged integrated circuit may then be connected to other components, e.g. on a printed circuit board by soldering.
Note that due to the reduced thickness of the tip portions 11 of the lead frame, there is a space underneath the tip portions 11 during the moulding process. This means that the resin is able to flow underneath the tip portions 11, thereby holding them securely in place and ensuring that the tip portions of the lead fingers 9 are covered on their lower surface. This makes it much easier to use the packaged die because there is much less chance of two lead fingers 9 (which are typically separated from each other by only 0.15 mm) being electrically connected to each other when the packaged integrated circuit is soldered to a printed circuit board.
A known problem with the above system is that during the wire bonding process the thin tips 11 of the fingers 9 may vibrate in a vertical direction despite the clamp-and-paddle arrangement. This is known as the “springboard” or “cantilever” effect, and can result in poor wire bonds to the tip portions 11. This is a contributing factor in lengthening the set-up time of the assembly process, and increasing the yield loss in the assembly process.
Various techniques have been suggested to overcome this problems, but all concentrate on improved clamping of the tip portions 11 of the fingers 9, by extra clamps located on the tip portions from above or below. However, such techniques assume that the lead frame 1 is very accurately positioned. In practice this is not the case, so the success of the packaging process is not consistent.